HORMA - 301
Growth And Development of Horticultural Crops
Growth And Development of Horticultural Crops
UNIT V: Growth & Development Under Stress and Manipulation Techniques
📚 Contents
1. Growth and Developmental Processes During Stress
Plants undergo major physiological and biochemical changes when exposed to abiotic or biotic stress. Stress affects cell division, cell elongation, photosynthesis, respiration, hormones, and assimilate partitioning.
1.1 Types of Stress and Their Effects
A. Water Stress (Drought)
- Physiological effects:
Stomatal closure → ↓ CO₂ uptake → ↓ photosynthesis
Reduced turgor → smaller leaves, slow cell expansion
Altered hormone levels: ↑ ABA, ↓ cytokinin
Increased reactive oxygen species (ROS) - Developmental consequences:
Reduced shoot growth
Poor flowering and fruit set
Increased flower and fruit drop
Accelerated senescence
B. Excess Water (Waterlogging)
- Reduces oxygen in root zone → anaerobic respiration
- Root decay, reduced nutrient uptake
- Ethylene accumulation → leaf yellowing, abscission
- Reduced root & shoot growth
C. Temperature Stress
| Heat Stress | Cold/Chilling Stress |
|---|---|
| Protein denaturation Membrane fluidity disruption Pollen sterility Floral abscission |
Photosystem damage Reduced membrane stability Delayed growth Fruit cracking in some crops |
D. Salinity Stress
- Ion toxicity (Na⁺, Cl⁻)
- Osmotic stress → reduced water uptake
- Yield reduction, smaller fruit size
- ↓ cell division, ↓ elongation
- Hormonal effect: ↑ ABA, ↓ GA and cytokinin
E. Nutrient Stress
Deficiency or toxicity alters: Photosynthesis, Meristem activity, Flowering & fruit set, Root-to-shoot ratio.
1.2 General Stress Responses
- A. Osmotic Adjustment: Accumulation of solutes (proline, sugars) to maintain turgor.
- B. Antioxidant Production: Enzymes: catalase, superoxide dismutase (SOD), peroxidase. Prevent oxidative damage.
- C. Changes in Hormones: ABA ↑ under drought, Ethylene ↑ under waterlogging, GA ↓ reduces growth under stress.
- D. Stress Proteins: Heat shock proteins (HSPs), Dehydrins (cold, drought response).
2. Manipulation of Growth and Development
Growth and development can be modified intentionally for improving yield, quality, or plant architecture.
2.1 Why Plants Are Manipulated?
- Improve light penetration
- Enhance fruiting and flowering
- Increase yield
- Reduce vegetative vigor
- Improve canopy structure
- Synchronize growth & harvest
- Enhance stress tolerance
3. Impact of Pruning and Training
Pruning = removal of plant parts.
Training = giving a plant a desired shape or structure.
Both directly manipulate growth, hormone distribution, light interception, and fruiting.
Training = giving a plant a desired shape or structure.
Both directly manipulate growth, hormone distribution, light interception, and fruiting.
3.1 Physiological Effects of Pruning
- A. Alters Hormone Balance: Removal of apical bud ↓ auxin → ↑ cytokinin → stimulates lateral shoot growth. Stimulates new vegetative growth.
- B. Redirects Assimilate Partitioning: More carbohydrates directed to remaining shoots and fruits. Improves fruit size and quality.
- C. Enhances Light Penetration: Better canopy aeration, Higher photosynthetic efficiency, Reduced disease incidence.
- D. Influences Flowering: Reduces excessive vegetative growth, Encourages reproductive growth in fruit crops. (Example: Spur pruning in grapes increases fruit bud formation).
3.2 Effects of Training
Training systems like open-center, central leader, modified leader, T-bar, pergola modify canopy structure.
- Benefits: Better sunlight distribution, Strong framework for bearing, Easier management, Higher fruit quality, Improved color development.
- Developmental Influence: Alters dominance relationships, Directs vegetative/reproductive balance, Influences bud differentiation.
4. Chemical Manipulations in Horticultural Crops
Plant growth regulators (PGRs) and chemicals modify growth patterns, flowering, fruiting, and ripening.
4.1 Chemicals Used for Growth Control
A. Growth Retardants
Reduce excessive vegetative growth.
- Examples: CCC (Cycocel / Chlormequat chloride), Paclobutrazol (PBZ), Daminozide (Alar).
- Effects: Short internodes, Reduced vegetative growth, Promotes flowering in mango, citrus, Controls plant height in ornamentals.
B. Growth Promoters
| Gibberellins (GA₃) | Breaks dormancy, Increases bunch length in grapes, Increases fruit size in grapes, strawberry. |
| Cytokinins | Delay senescence, Enhance shoot proliferation, Improve flower quality. |
| Auxins (IAA, NAA, IBA) | Root induction, Reduces fruit drop, Controls apical dominance. |
4.2 Chemicals for Flower Induction
- Ethephon stimulates flowering in pineapple.
- KNO₃ used in mango flowering under off-season conditions.
4.3 Chemicals for Fruit Retention
- NAA reduces early fruit drop in citrus.
- 2,4-D reduces pre-harvest drop in mango.
4.4 Ripening and Color Development
- Ethylene / Ethephon induces ripening in climacteric fruits.
- Ethrel improves fruit color (e.g., citrus, tomato).
4.5 Chemicals for Dormancy Breaking
- Hydrogen cyanamide (Dormex) used in grapes, kiwi to break bud dormancy.
5. Molecular and Genetic Approaches
Modern horticulture increasingly uses molecular tools to improve growth, stress tolerance, flowering, and fruit quality.
5.1 Gene Manipulation for Stress Tolerance
| Drought tolerance | DREB, CBF, AREB/ABF → improve water stress tolerance. |
| Heat tolerance | HSPs (Heat shock proteins), HsfA1 regulators. |
| Salinity tolerance | SOS1, SOS2, SOS3 – ion homeostasis, NHX1 – vacuolar Na⁺ sequestration. |
5.2 Flowering Regulation Genes
- FT (Florigen gene) – controls flowering induction.
- LFY, AP1 – floral meristem identity.
- CO (CONSTANS) – photoperiodic pathway.
By modifying these genes → early or late flowering cultivars can be developed.
5.3 Manipulating Fruit Development
- Seedless fruits: Knockdown of seed development genes (e.g., RNAi for seedless tomato).
- Delayed ripening: ACO gene silencing (ethylene synthesis enzyme). Used in tomato (FlavrSavr).
- Enhanced nutrition: Overexpression of carotenoid genes → ↑ lycopene, β-carotene (Biofortification).
5.4 Tissue Culture & Micropropagation
Molecular tools + tissue culture allow: Mass propagation, Production of disease-free plants, Somaclonal variation for new traits, Embryo rescue, haploid breeding.
5.5 CRISPR/Cas Gene Editing
Used for: Disease resistance, Abiotic stress tolerance, Flowering time control, Compact growth habit, Improving fruit quality.
Advantages: Precise, Faster breeding cycles, No foreign DNA in many cases (non-GMO classification in some countries).
Advantages: Precise, Faster breeding cycles, No foreign DNA in many cases (non-GMO classification in some countries).
📝 Summary of Unit V
- Stress affects growth through hormones, metabolism, and cell activity.
- Pruning & training modify physiological balance and improve productivity.
- Chemicals regulate growth, flowering, fruiting, ripening, dormancy.
- Molecular methods now central to horticulture for improving development & tolerance.
📚 REFERENCES
Buchanan B, Gruissem W & Jones R. 2002. Biochemistry & Molecular Biology of Plants. Wiley.
Epstein E. 1972. Mineral Nutrition of Plants. Wiley.
Fosket D.E. 1994. Plant Growth and Development. Academic Press.
Leopold A.C. & Kriedemann P.E. 1985. Plant Growth and Development. McGraw-Hill.
Salisbury F.B. & Ross C.W. 1992. Plant Physiology. 4th Ed. Wadsworth Publ.
Peter K.V. 2008. Basics of Horticulture. New India Publishing Agency.
Roberts J.A., Downs S.J., Parker P. 2002. In Plants (I. Ridge, Ed.). Oxford Univ. Press.